Steel composition

ABSTRACT

It further relates to method of manufacture thereof, the steel blank obtained and a mechanical device or an injection system comprising same.

The present invention relates to a new steel of grade 10CrMoNiVCo withlow carbon content and high cobalt content for thermochemical treatmentin particular intended for the field of transmissions such as bearingsand gears. The alloy according to the invention is also usable for otherapplications requiring high surface hardness combined with good coretoughness, for example in the case of injection systems.

Bearings are mechanical devices allowing relative movements, constrainedin orientation and direction, between two components. Bearings compriseseveral components: inner race, outer race as well as rolling bodies(balls or rollers) arranged between these two races. To ensurereliability and performance over time, it is important that thesevarious elements have good properties of rolling fatigue, wear, etc.

Gear trains are mechanical devices for power transmission. To ensure afavorable power density (ratio of power transmitted to overalldimensions of the gear train) and operational reliability, gear trainsmust have good properties of structural fatigue (tooth root) and contactfatigue (tooth flank).

The conventional techniques for producing these metallic componentsemploy production methods of electric steelmaking followed by optionaloperations of remelting, or single or multiple vacuum remelting. Theingots thus produced are then formed by methods of hot working such asrolling or forging in the form of bar, tube or rings.

There are two types of metallurgy for providing the final mechanicalproperties.

1st Type: the chemical composition of the component makes it possible toobtain the mechanical properties directly after suitable heat treatment.

2nd Type: the component requires a thermochemical treatment to enrichthe surface with interstitial chemical elements such as carbon and/ornitrogen. This enrichment, generally at the surface, then allows highmechanical properties to be obtained after heat treatment to depths ofsome millimeters at most. These steels generally have better propertiesof ductility than the steels of the 1st type.

There are also thermochemical methods applied to steels of the 1st typewith the aim of enriching the surface with nitrogen to obtain very highmechanical properties.

The first of the properties required in the field of bearings or geartrains is obtaining a very high level of hardness. These steels of type1 and of type 2 generally have levels of surface hardness above 58 HRC.The grades used most widely and known by the term M50 (0.8% C-4% Cr-4.2%Mo-1% V) or 50NiL (0.12% C-4% Cr-4.2% Mo-3.4% Ni-1% V) do not exceed,after optional thermochemical treatment and suitable heat treatment, asurface hardness of 63 HRC. It is now necessary to obtain hardnessesabove 64 HRC for significant improvement of the properties of thecomponent.

Application GB2370281 describes a valve seat steel produced by powdermetallurgy technology starting from mixtures of powder with an iron baseand harder particles. The matrix, which only constitutes one part of thesteel, has the following composition, in percentages by weight of thetotal composition:

-   -   Carbon: 0.2-2.0;    -   Chromium: 1.0-9.0;    -   Molybdenum: 1.0-9.0;    -   Silicon: 0.1-1.0;    -   Tungsten: 1.0-3.0;    -   Vanadium: 0.1-1.0;    -   Nickel+Cobalt+Copper: 3.0-15.0;    -   Iron: remainder

However, this matrix comprises from 5 to 40 vol % of pearlite, withconsequent lack of ductility of this matrix and therefore embrittlement.Furthermore, the material also contains porosity (up to 10%), which doesnot allow good properties of mechanical strength and fatigue strength tobe achieved. Finally, this document does not suggest using a low coppercontent and on the contrary indicates that its content may be up to 15wt %. Now, a high copper content is undesirable for the applications ofthe present invention as copper is known to cause embrittlement and itscontent should not exceed 0.5 wt % relative to the total weight of thecomposition of the steel.

Patent application WO2015/082342 describes a rolling bearing steelhaving the following composition, in percentages by weight of the totalcomposition:

-   -   Carbon: 0.05-0.5;    -   Chromium: 2.5-5.0;    -   Molybdenum: 4-6;    -   Tungsten: 2-4.5;    -   Vanadium: 1-3;    -   Nickel: 2-4;    -   Cobalt: 2-8;    -   Iron: remainder

as well as the inevitable impurities, optionally further comprising oneor more of the following elements:

-   -   Niobium: 0-2;    -   Nitrogen: 0-0.5;    -   Silicon: 0-0.7;    -   Manganese: 0-0.7;    -   Aluminum: 0-0.15;

and in particular grade MIX5 of composition (0.18% C-3.45% Cr-4.93%Mo-3.05% W-2.09% V-0.30% Si-2.89% Ni-5.14% Co-0.27% Mn), which is themost interesting as it has the highest surface hardness. This grademakes it possible to reach a surface hardness after solution heattreatment at 1150° C. and tempering at 560° C. at a maximum hardnesslevel of about 800 HV, or the equivalent of max. 64 HRC. However, thisapplication states that the Co content must be limited to at most 8% andit is even preferable for it to be at most 7% and even more preferablyat most 6% as Co increases the level of hardness of the base material,which leads to a decrease in toughness. The grade MIX5 that is preferredthus has a Co content of 5.14%.

Patent application WO2017216500 describes a rolling bearing steel havingthe following composition, in percentages by weight of the totalcomposition:

-   -   Carbon: 0.05-0.40, preferably 0.10-0.30;    -   Chromium: 2.50-5.00, preferably 3.0-4.5;    -   Molybdenum: 4.0-6.0;    -   Tungsten: 0.01-1.8, preferably 0.02-1.5;    -   Vanadium: 1.0-3.0, preferably 1.5-2.5;    -   Nickel: 2.0-4.0;    -   Cobalt: 2.0-8.0, preferably 3.0-7.0;    -   Iron: remainder

as well as the inevitable impurities,

optionally further comprising one or more of the following elements:

-   -   Niobium: ≤2.0;    -   Nitrogen: ≤0.50, preferably ≤0.20;    -   Silicon: ≤0.70, preferably 0.05-0.50;    -   Manganese: ≤0.70, preferably 0.05-0.50;    -   Aluminum: ≤15, preferably ≤0.10;

the combined niobium+vanadium content being in the range 1.00-3.50; andthe carbon+nitrogen content being in the range 0.05-0.50.

In particular, in the examples, grade C of composition (0.18-0.20%C-3.90-4.00% Cr-5.00-5.20% Mo-0.10-0.20% W-2.10-2.30% V-0.14-0.16%Si-3.05-3.09% Ni-5.00-5.40% Co-0.18-0.22% Mn-0.03-0.05% Al) ispreferred, as it has the highest surface hardness. This grade makes itpossible to reach a surface hardness after solution heat treatment at1100° C.-1150° C. and tempering at 500° C. at a maximum level ofhardness of about 66-67 HRC, which is well above the surface hardnessobtained with a grade according to application WO2015/082342 (grade A:FIG. 1). However, this application also states that the Co content mustbe limited to at most 8% and it is even preferable for it to be at most7% and even more preferably at most 6% as it increases the level ofhardness of the base material, which leads to a decrease in toughness.The grade C that is preferred thus has a Co content of 5.00-5.40%.

Obtaining surface hardnesses above 67 HRC, in particular using asolution heat treatment at a temperature less than or equal to 1160° C.,is therefore difficult to achieve, whereas they would allow significantimprovement of the properties of the component.

The inventors found, surprisingly, that by increasing the cobalt contentof the steel described in applications WO2015/082342 and WO2017216500 toa content between 9 and 12.5%, while maintaining the carbon content at alevel less than or equal to 0.2% (new carbon/cobalt balance), the steelobtained had, after thermochemical treatment, in particular carburizingand/or nitriding, a very high surface hardness, even above 67 HRC, inparticular greater than or equal to 68 HRC and a hardness at 1 mmgreater than 860 HV (which corresponds to about 66 HRC according tostandard ASTME140-12b published in May 2013) after solution heattreatment at a temperature in the range 1100° C.-1160° C. and temperingat a temperature greater than or equal to 475° C., while displaying alevel of hardness of the base material between 400 and 650 HV.

This was not at all obvious in view of these documents, which suggestedusing a lower cobalt content such as in grade MIXS (5.14% of cobalt) andin grade C (5.00-5.40% of cobalt), which are regarded as thecompositions giving the best hardness.

U.S. Pat. No. 8,157,931 describes a steel of type Ni—Co having a cobaltcontent between 9.9 and 10% and a carbon content between 0.1 and 0.12%and having a high surface hardness of the order of 68-69 HRC. However,said steel has a high chromium content (5.3-5.4%), a low content ofvanadium (0.20-0.21%) and molybdenum (2.5-2.52%) and does not containtungsten. This grade balancing leads, after thermochemical treatment andassociated quality treatment (comprising quenching at 1110° C. andtempering at 482° C.), to a surface hardness that is interesting, butdecreases very quickly with depth, thus starting from 600 μm of depth itis already identical to that of the base metal (FIG. 1). This isprobably due to the lower carbon content in the cemented layer that thegrade can support to avoid any risk of formation of embrittling graphitephase. Claim 1 of that patent thus stipulates a carbon content in thecemented layer limited to about 0.8%. In fact, graphite could appearstarting from 1 wt % of C in the cemented layer (surface layer obtainedafter carburizing).

It is therefore not obvious to find good balancing of the grade(including Cr, Mo, V, W, C) in view of this document to achievesimultaneous optimization of surface hardness, hardness profile (depth)and toughness (for which we have an idea from the core hardness).Furthermore, it was not obvious in view of this document to produce adeep carburizing layer that would allow much more carbon to beintroduced than the grades of the prior art (up to 1.5 wt % of C) whilelimiting the risk of appearance of graphite.

Patent application JPH11-210767 describes a class of steel foraeronautical rolling bearing application with an improved service lifehaving the following composition, in percentages by weight of the totalcomposition:

-   -   Carbon: max. 0.05;    -   Chromium: 2.5-5.5;    -   Tungsten equivalent (2×Mo+W): 12.5-20;    -   Vanadium: max. 1.5;    -   Nickel: max. 5.0;    -   Cobalt: max. 20.0;    -   Silicon: 0.15-1.0    -   Manganese: 0.15-1.5    -   Iron: remainder

This grade is submitted to carburizing or carbonitriding.

However, this application only describes the properties of surfacehardness of 66-69 HRC and only describes the toughness qualitatively.Balancing of this grade at very low carbon, 0.05 wt %, necessitateslimiting the vanadium content to 1.5 wt % so as not to degrade thetoughness, vanadium being an interesting element allowing wearresistance to be improved.

Moreover, this application does not describe the core hardness(reflecting the mechanical strength) of this grade, and in view of thevery low level of carbon, this is expected to degrade the mechanicalstrength.

Furthermore, this application does not describe any carburizing profileto a deep layer. Now, it would be interesting to have high hardness inthe full depth as far as 400 microns from the surface, which correspondsto the so-called Hertz zone, a zone subjected to very high shearstresses. High hardness throughout this depth also provides moretolerance when it comes to removing material for repair or grindingduring machining, and this is all the more useful for the powertransmission application, which is not mentioned in JPH11-210767.

The inventors realized that it was possible to obtain balancingdifferent from that proposed by JPH11-210767 with a higher carboncontent, at least 0.06 wt %, and a range of cobalt between 9.0 and 12.5wt %, making it possible (a) to obtain a good compromise between corehardness and toughness, in other words a good compromise betweenmechanical strength and toughness, and (b) to allow more vanadium in itscomposition without degrading the toughness, which is favorable for wearresistance.

The present invention therefore relates to a steel composition,advantageously carburizable and/or nitridable, more advantageouslycarburizable, comprising, advantageously consisting essentially of, inparticular consisting of, in percentages by weight of the totalcomposition:

-   -   Carbon: 0.06-0.20 preferably 0.08-0.18;    -   Chromium: 2.5-5.0, preferably 3.0-4.5;    -   Molybdenum: 4.0-6.0;    -   Tungsten: 0.01-3.0;    -   Vanadium: 1.0-3.0, preferably 1.50-2.50;    -   Nickel: 2.0-4.0;    -   Cobalt: 9.0-12.5, preferably 9.5-11.0;    -   Iron: remainder

as well as the inevitable impurities,

optionally further comprising one or more of the following elements:

-   -   Niobium: ≤2.0;    -   Nitrogen: ≤0.50, preferably ≤0.20;    -   Silicon: ≤0.70, preferably 0.05-0.50;    -   Manganese: ≤0.70, preferably 0.05-0.50;    -   Aluminum: ≤0.15, preferably ≤0.10;

the combined niobium+vanadium content being in the range 1.0-3.5; andthe carbon+nitrogen content being in the range 0.06-0.50.

A particularly interesting composition comprises, advantageouslyconsists essentially of, in particular consists of, in percentages byweight of the total composition:

-   -   Carbon: 0.06-0.20, preferably 0.08-0.18;    -   Chromium: 3.0-4.5, preferably 3.5-4.5;    -   Molybdenum: 4.0-6.0, preferably 4.5-5.5;    -   Tungsten 0.01-3.0;    -   Vanadium: 1.5-2.5, preferably 2.0-2.3;    -   Nickel: 2.0-4.0, preferably 2.5-3.5;    -   Cobalt: 9.5-12.5, preferably 9.5-10.5;    -   Iron: remainder

as well as the inevitable impurities,

optionally further comprising one or more of the following elements:

-   -   Niobium: ≤2.0;    -   Nitrogen: ≤0.20;    -   Silicon: ≤0.70, preferably 0.05-0.50;    -   Manganese: ≤0.70, preferably 0.05-0.50;    -   Aluminum: ≤0.10;

the combined niobium+vanadium content being in the range 1.00-3.50; andthe carbon+nitrogen content being in the range 0.06-0.50.

In particular, the inevitable impurities, notably selected from titanium(Ti), sulfur (S), phosphorus (P), copper (Cu), tin (Sn), lead (Pb),oxygen (O) and mixtures thereof, are kept at the lowest level. Theseimpurities are generally due essentially to the method of manufactureand the quality of the charge. Advantageously, the composition accordingto the invention comprises at most 1 wt % of inevitable impurities,advantageously at most 0.75 wt %, even more advantageously at most 0.50wt %, relative to the total weight of the composition.

The carbide formers, which also have a stabilizing effect on ferrite,so-called alpha-forming elements, are essential to the steel compositionaccording to the invention so as to provide sufficient hardness, heatresistance and wear resistance. In order to obtain a microstructure freefrom ferrite, which would weaken the component, it is necessary to addaustenite stabilizers, so-called gamma-forming elements.

A correct combination of austenite stabilizers (carbon, nickel, cobaltand manganese) and ferrite stabilizers (molybdenum, tungsten, chromium,vanadium and silicon) makes it possible to obtain a steel compositionaccording to the invention having superior properties, in particularafter thermochemical treatment such as carburizing.

The steel composition according to the invention therefore comprisescarbon (C) at a content in the range 0.06-0.20%, preferably 0.07-0.20%,in particular 0.08-0.20%, more particularly 0.08-0.18%, by weightrelative to the total weight of the composition. In fact carbon (C)stabilizes the austenitic phase of the steel at the heat treatmenttemperatures and is essential for formation of carbides, which supplythe mechanical properties in general, notably mechanical strength, highhardness, heat resistance and wear resistance. The presence of a smallamount of carbon in a steel is beneficial for avoiding formation ofundesirable, brittle intermetallic particles and for forming smallamounts of carbides to avoid excessive grain growth during solutiontreatment before the quenching operation. The initial carbon contentneed not, however, be too high, since it is possible to increase thesurface hardness of the components formed from the steel composition bycarburizing. It is also known that, generally, increasing the carboncontent makes it possible to increase the level of hardnesssignificantly, which is generally detrimental with respect to theductility properties. That is why the carbon content is limited to max.0.20% to obtain a level of core hardness of the material of at most 650HV. During carburizing, carbon is introduced into the surface layers ofthe component, so as to obtain a hardness gradient. Carbon is theprincipal element for controlling the hardness of the martensitic phaseformed after carburizing and heat treatment. In a case-hardened steel,it is essential to have a core portion of the material with a low carboncontent while having a hard surface with a high carbon content aftercarburizing thermochemical treatment.

The steel composition according to the invention further compriseschromium (Cr) at a content in the range 2.5-5.0%, preferably 3.0-4.5%,even more preferably 3.5-4.5%, even more advantageously 3.8-4.0 wt %relative to the total weight of the composition.

Chromium contributes to the formation of carbides in steel and is one ofthe main elements controlling the hardenability of steels.

However, chromium may also promote the appearance of ferrite andresidual austenite. Therefore the chromium content of the steelcomposition according to the invention must not be too high.

The steel composition according to the invention also comprisesmolybdenum (Mo) at a content in the range 4.0-6.0%, preferably 4.5-5.5%,even more preferably 4.8-5.2%, by weight relative to the total weight ofthe composition.

Molybdenum improves tempering resistance, wear resistance and thehardness of steel. However, molybdenum has a strong stabilizing effecton the ferrite phase and therefore should not be present in an excessiveamount in the steel composition according to the invention.

The steel composition according to the invention further comprisestungsten (W) at a content in the range 0.01-3.0%, preferably 0.01-1.5%,even more preferably 0.01-1.4%, advantageously 0.01-1.3%, by weightrelative to the total weight of the composition.

Tungsten is a ferrite stabilizer and a strong carbide former. Itimproves resistance to heat treatment and to wear as well as hardness byforming carbides. However, it may also lower the surface hardness of thesteel and especially the properties of ductility and toughness. For thiselement to perform its role fully, it is necessary to apply solutiontreatment at high temperature.

The steel composition according to the invention further comprisesvanadium (V) at a content in the range 1.0-3.0%, preferably 1.5-2.5%,even more preferably 1.7-3.0%, advantageously 1.7-2.5%, moreadvantageously 1.7-2.3%, even more advantageously 2.00-2.3%, inparticular 2.0-2.2%, by weight relative to the total weight of thecomposition.

Vanadium stabilizes the ferrite phase and has a strong affinity forcarbon and nitrogen. Vanadium provides wear resistance and temperingresistance by forming hard vanadium carbides. Vanadium may be replacedpartly with niobium (Nb), which has similar properties.

The combined niobium+vanadium content must therefore be in the range1.0-3.5 wt % relative to the total weight of the composition,advantageously in the range 1.7-3.5 wt % relative to the total weight ofthe composition.

If niobium is present, its content must be 2.0 wt % relative to thetotal weight of the composition. Advantageously, the steel compositionaccording to the invention does not comprise niobium.

The steel composition according to the invention also comprises nickel(Ni) at a content in the range 2.0-4.0%, preferably 2.5-3.5%, even morepreferably 2.7-3.3%, advantageously 3.0-3.2%, by weight relative to thetotal weight of the composition.

Nickel promotes the formation of austenite and therefore inhibits theformation of ferrite. Another effect of nickel is to lower thetemperature Ms, i.e. the temperature at which the transformation ofaustenite to martensite begins during cooling. This may preventmartensite formation. The amount of nickel must therefore be controlledso as to avoid formation of residual austenite in the carburizedcomponents.

The steel composition according to the invention further comprisescobalt (Co) at a content in the range 9.0-12.5%, preferably 9.5-12.5%,advantageously 9.5-11.0%, more advantageously 9.5-10.5%, by weightrelative to the total weight of the composition. The cobalt content ismeasured according to standards ASTM-E1097-12 published in June 2017 andASTM E1479_16 published in December 2016. The error in measurement ofthe cobalt content of the steel according to the invention is thus about±2.5% relative, and is evaluated according to standards IS05724-1(December 1994), ISO5725-2 (December 1994), ISO5725-3 (December 1994),ISO5725-4 (December 1994), ISO5725-5 (December 1994), ISO5725-6(December 1994) and standard NF ISO/CEI Guide 98-3 of 11 Jul. 2014.

Cobalt is a strong austenite stabilizer that prevents the formation ofundesirable ferrite. In contrast to nickel, cobalt increases thetemperature Ms, which in its turn decreases the amount of residualaustenite. Cobalt, in combination with nickel, allows the presence offerrite stabilizers such as the carbide formers Mo, W, Cr and V. Thecarbide formers are essential for the steel according to the inventionon account of their effect on hardness, heat resistance and wearresistance. Cobalt has a small effect on the steel of increasing thehardness. However, this increase in hardness is correlated with adecrease in toughness. Therefore the steel composition according to theinvention should not contain an excessive amount of cobalt. Addition ofCo makes it possible to limit the content of C, avoiding the promotionof ferrite for a composition according to the invention (containing thecontents of Cr, Mo, V, Ni and W as described above). This limitation ofcarbon makes it possible to compensate for the increase in hardnessassociated with the addition of Co.

The steel composition according to the invention may further comprisesilicon (Si) in a content ≤0.70 wt % relative to the total weight of thecomposition. Advantageously, it comprises silicon, in particular at acontent in the range 0.05-0.50%, preferably 0.05-0.30%, advantageously0.07-0.25%, even more advantageously 0.10-0.20%, by weight relative tothe total weight of the composition.

Silicon is a strong ferrite stabilizer, but is often present duringsteelmaking, during deoxidation of the molten steel. Low oxygen contentsare in fact also important for obtaining low levels of nonmetallicinclusions and good mechanical properties such as fatigue strength andmechanical strength.

The steel composition according to the invention may further comprisemanganese (Mn) in a content ≤0.70 wt % relative to the total weight ofthe composition. Advantageously, it comprises manganese, in particularat a content in the range 0.05-0.50%, preferably 0.05-0.30%,advantageously 0.07-0.25%, even more advantageously 0.10-0.22%, evenmore particularly 0.10-0.20% by weight relative to the total weight ofthe composition.

Manganese stabilizes the austenite phase and decreases the temperatureMs in the steel composition. Manganese is generally added to the steelsduring their manufacture owing to its affinity for sulfur, there is thusformation of manganese sulfide during solidification. This eliminatesthe risk of formation of iron sulfides, which have an unfavorable effecton hot machining of the steels. Manganese also forms part of thedeoxidation step, like silicon. The combination of manganese and silicongives more effective deoxidation than each of these elements alone.

Optionally, the steel composition according to the invention maycomprise nitrogen (N), in a content ≤0.50%, preferably ≤0.20%, by weightrelative to the total weight of the composition.

Nitrogen promotes austenite formation and lowers the transformation ofaustenite to martensite. Nitrogen may to a certain extent replace carbonin the steel according to the invention, forming nitrides. However, thecarbon+nitrogen content must be in the range 0.06-0.50 wt % relative tothe total weight of the composition.

Optionally, the steel composition according to the invention maycomprise aluminum (Al), in a content ≤0.15%, preferably ≤0.10%, byweight relative to the total weight of the composition.

Aluminum (Al) may in fact be present during steelmaking according to theinvention and contributes very effectively to deoxidation of the moltensteel. This is the case in particular in remelting processes, such asthe VIM-VAR process. The aluminum content is in general higher in thesteels produced by the VIM-VAR process than in the steels obtained bypowder metallurgy. Aluminum gives rise to difficulties duringatomization by obstructing the pouring spout with oxides.

A low oxygen content is important for obtaining good micro-cleanness aswell as good mechanical properties such as fatigue strength andmechanical strength. The oxygen contents obtained by the ingot route aretypically below 15 ppm.

Advantageously, the composition according to the present invention iscarburizable, i.e. it can undergo a carburizing treatment, and/ornitridable, i.e. it can undergo a nitriding treatment and evenadvantageously it can undergo a thermochemical treatment, in particularselected from carburizing, nitriding, carbonitriding and carburizingfollowed by nitriding. These treatments make it possible to improve thesurface hardness of the steel, by adding the elements carbon and/ornitrogen. Thus, if carburizing is used, the carbon content of thesurface of the steel increases and therefore leads to an increase insurface hardness. The surface (surface layer advantageously having athickness of 100 microns) is thus advantageously enriched with carbon toobtain a final carbon content (final surface carbon content) of 0.5%-1.7wt %, more particularly of 0.8%-1.5 wt %, more advantageously of atleast 1 wt %, in particular of 1-1.3 wt %, even more advantageously >1.1wt %, even more particularly between 1.2 and 1.5 wt %. In the rest ofthis document, the surface carbon content will be understood to havebeen determined by sampling from a surface layer to a depth of 100microns.

If nitriding is used, it is the nitrogen content that increases at thesurface of the steel, and therefore also the surface hardness.

If carbonitriding or carburizing followed by nitriding is used, it isthe contents of carbon and nitrogen at the surface of the steel that areincreased and therefore also the surface hardness.

These methods are familiar to a person skilled in the art.

In an advantageous embodiment, the steel composition according to theinvention has, after a thermochemical treatment, advantageously ofcarburizing or of nitriding or of carbonitriding or of carburizing andthen nitriding, followed by a heat treatment, a surface hardness above67HRC, in particular greater than or equal to 68 HRC, measured accordingto standard ASTM E18 published in July 2017 or an equivalent standard.It also has, advantageously, a surface hardness greater than or equal to910 HV (about 67.25 HRC according to standard ASTM E140-12b published inMay 2013), advantageously greater than or equal to 920 HV, in particulargreater than or equal to 940 HV, measured according to standard ASTME384 published in August 2017 or an equivalent standard, in particularafter a solution treatment at a temperature of 1100° C. It also has,advantageously, a surface hardness greater than or equal to 930 HV(corresponding to about 67.75 HRC according to standard ASTM E140-12bpublished in May 2013), advantageously greater than or equal to 940 HV(corresponding to 68 HRC according to standard ASTM E140-12b publishedin May 2013), in particular greater than or equal to 950 HV, measuredaccording to standard ASTM E384 published in August 2017 or anequivalent standard after a solution treatment at a temperature of 1150°C.

It also has, advantageously, a hardness at a depth of 1 mm greater thanor equal to 860 HV (which corresponds to about 66 HRC according tostandard ASTM E140-12b published in May 2013), advantageously greaterthan or equal to 870 HV, in particular greater than or equal to 880 HV,measured according to standard ASTM E384 published in August 2017 or anequivalent standard, in particular after a solution treatment at atemperature of 1100° C. It also has, advantageously, a hardness at adepth of 1 mm greater than or equal to 880 HV, advantageously greaterthan or equal to 890 HV, in particular greater than or equal to 900 HV,measured according to standard ASTM E384 published in August 2017 or anequivalent standard.

It also has, advantageously, a level of hardness of the base material(core material hardness) between 440 and 650 HV, advantageously between440 and 630 HV, measured according to standard ASTM E384 published inAugust 2017 or an equivalent standard.

The steel composition obtained as a result of these treatmentsadvantageously has a surface concentration of carbon (final surfacecontent) of 1-1.3 wt %.

Said heat treatment may comprise:

-   -   (1) solution treatment of the steel at a temperature between        1090° C.-1160° C., advantageously between 1100° C.-1160° C.,        more advantageously between 1100 and 1155° C., in particular        between 1100 and 1150° C., more particularly of 1150° C.,    -   (2) advantageously followed by holding at this temperature until        completion of austenitization, in particular for 15 minutes        (quenching), (these 2 steps (1) and (2) allow complete or        partial solution of the carbides initially present),    -   (3) and then optionally a first cooling (quenching), in        particular under neutral gas, for example at a pressure of 2 bar        (2×10⁵ Pa), advantageously to room temperature (this step makes        it possible to obtain a mainly martensitic microstructure with        residual austenite. This residual austenite is a function of the        cooling temperature: the content decreases with the cooling        temperature),    -   (4) optionally followed by holding at room temperature,    -   (5) and then advantageously a second cooling to a temperature        below −40° C., more advantageously below −60° C., even more        advantageously of about −70° C., in particular for 2 hours (this        step makes it possible to decrease the content of residual        austenite),    -   (6) and advantageously one or more tempering operations, more        advantageously at least three tempering operations,        advantageously at a temperature greater than or equal to 475°        C., more advantageously between 475° C. and 530° C., in        particular of 500° C., even more particularly for 1 hour each        (this or these tempering operation(s) allow precipitation of        carbides and partial or complete decomposition of the residual        austenite. This makes it possible to obtain properties of        ductility).

The advantage of the steel according to the invention is therefore thatof obtaining high levels of hardness with a limited heat treatment(temperature between 1090° C.-1160° C., advantageously between 1100°C.-1160° C., more advantageously between 1100° C.-1155° C., inparticular between 1100° C.-1150° C., more particularly of 1150° C.).

In a particularly advantageous embodiment, the steel compositionaccording to the invention has, after thermochemical treatment,advantageously of carburizing or of nitriding or of carbonitriding or ofcarburizing and then nitriding, followed by a heat treatment, amartensitic structure having a residual austenite content below 10 wt %,more advantageously below 0.5 wt %, and free from ferrite and pearlite,phases that are known to decrease the surface hardness of steel. Saidheat treatment may be as described above.

The present invention further relates to a method of manufacturing asteel blank having the composition according to the invention,characterized in that it comprises:

a) a steelmaking step;

b) a step of transformation of the steel;

c) a thermochemical treatment;

d) and a heat treatment.

Advantageously the heat treatment in step d) of the method according tothe present invention is as described above.

Advantageously, the thermochemical treatment in step c) of the methodaccording to the present invention consists of a treatment ofcarburizing or of nitriding or of carbonitriding or of carburizing andthen nitriding, advantageously it is a carburizing treatment, moreparticularly allowing carbon enrichment of the surface, leading to afinal surface carbon content of at least 1 wt %, even moreadvantageously >1.1 wt %.

In particular, step b) of the method according to the present inventionconsists of a step of rolling, forging and/or extrusion, advantageouslyforging. These methods are familiar to a person skilled in the art.

In an advantageous embodiment, the steelmaking step a) of the methodaccording to the present invention is carried out by a conventionalsteelmaking process in an arc furnace with refining and remelting underconductive slag (ESR, electroslag remelting), or by a VIM or VIM-VARprocess, optionally with a step of remelting under conductive slag (ESR,electroslag remelting) and/or under vacuum (VAR), or by powdermetallurgy such as gas atomization and compaction by hot isostaticpressing (HIP).

Thus, the steel according to the present invention may be produced by aVIM-VAR process. This process makes it possible to obtain very goodcleanness with respect to inclusions, and improves the chemicalhomogeneity of the ingot. It is also possible to proceed by a route ofremelting under conductive slag (ESR: ElectroSlag Remelting) or tocombine ESR and VAR (vacuum remelting) operations.

This steel may also be obtained by powder metallurgy. This method makesit possible to produce metal powder of great purity by atomization,preferably gas atomization to obtain low oxygen contents. The powder isthen compacted for example by hot isostatic pressing (HIP).

These methods are familiar to a person skilled in the art.

The present invention also relates to a steel blank obtainable by themethod according to the invention. This blank is made on the basis ofsteel having the composition according to the present invention and asdescribed above.

It further relates to the use of a blank according to the invention orof a steel composition according to the invention for making amechanical device or an injection system, advantageously a transmissioncomponent such as a gear train, a transmission shaft and/or a rollingbearing and in particular a rolling bearing.

It thus relates to a mechanical device, advantageously a transmissioncomponent, in particular a gear train, a transmission shaft or abearing, more particularly a bearing or a gear train, even moreparticularly a bearing, made of steel having the composition accordingto the invention or obtained from a steel blank according to theinvention.

It finally relates to an injection system made of steel having thecomposition according to the invention or obtained from a steel blankaccording to the invention.

In fact, with the steel composition according to the invention, it ispossible to combine high surface hardness and resistance to surface wearafter thermochemical treatment with a core portion of the materialhaving a high fatigue strength and a high mechanical strength.

These steels are therefore usable in demanding fields such as bearingsfor aerospace applications or injection systems.

The invention will be better understood on reading the followingexamples, which are given as a guide and are nonlimiting.

In the examples, unless stated otherwise, all the percentages areexpressed by weight, the temperature is expressed in degrees Celsius andthe pressure is atmospheric pressure.

1ST SERIES OF EXAMPLES

Seven laboratory heats of about 9 kg each (6 examples according to theinvention and a comparative example with a composition similar to thatin U.S. Pat. No. 8,157,931: comparative example 1) were produced by theVIM process according to the composition shown in Table 1 below (in wt %relative to the total weight of the composition), the remainder beingFe:

TABLE 1 Element C Ni Cr Mo V W Co Si Mn Al N Example 1: 0.18 3.1 3.9 5.12.1 1.18 10.0 0.2 0.18 0.023 0.005 GRADE A Example 2: 0.20 3.1 3.9 5.12.2 2.96 10.1 0.18 0.21 0.02 0.009 GRADE B Example 3: 0.16 3.1 3.9 5.12.1 1.19 10.0 0.21 0.18 0.02 0.009 GRADE C Example 4: 0.16 3.0 4.0 5.12.1 2.92 10.1 0.22 0.25 0.016 0.005 GRADE D Example 5: 0.16 3.1 3.9 5.02.1 0.01 10.0 0.123 0.2 0.042 0.005 GRADE E Example 6: 0.17 3.1 4.0 5.22.2 0.01 12.4 0.17 0.2 0.038 0.006 GRADE F Comparative 0.14 3.1 2.1 2.71.2 1.32 10.0 0.222 0.16 0.022 0.004 example 1: GRADE G

The Nb content is below the limit of detection. Nb<0.005% for all theexamples.

These compositions are very similar, with the exception of comparativeexample 1. The notable main differences between comparative example 1and example 1 relate to the content of V, Mo and Cr.

These laboratory heats were transformed into bars with a diameter of 40mm by hot forging with a 2000 T press. Rods with a diameter of 20 mmwere machined from the bar and carburized.

The carburized rods were treated by (1) a solution treatment at 1100° C.or 1150° C., (2) holding at this temperature for 15 min foraustenitization, (3) cooling under neutral gas at a pressure between 2and 6 bar (2×10⁵ and 6×10⁵ Pa), (4) a period at room temperature, (5)cooling to −70° C. for 2 hours, and (6) 3 tempering operations at atemperature of 500° C. for 1 hour each.

The profiles of surface hardness in HV measured according to standardASTM E384 published in August 2017 for examples 1 to 6 and comparativeexample 1 are presented in Tables 2 and 3.

TABLE 2 (solution treatment at 1100° C.) Core Hardness material at depthSurface Example hardness of 1 mm hardness Example 1: GRADE A 522 888 936Example 2: GRADE B 485 863 927 Example 3: GRADE C 542 890 938 Example 4:GRADE D 495 878 934 Example 5: GRADE E 554 880 942 Example 6: GRADE F567 927 976 Comparative example 1: 576 835 847 GRADE G

TABLE 3 (solution treatment at 1150° C.) Core Hardness material at depthSurface Example hardness of 1 mm hardness Example 1: GRADE A 550 888 949Example 2: GRADE B 543 888 943 Example 3: GRADE C 603 933 957 Example 4:GRADE D 552 904 957 Example 5: GRADE E 612 934 940 Example 6: GRADE F627 936 988 Comparative example 1: 585 868 878 GRADE G

For all the chemical compositions except comparative example 1, thesurface hardness after carburizing exceeds 920 HV for a temperature ofsolution treatment of 1100° C. and exceeds 930 HV for a temperature ofsolution treatment of 1150° C. The hardness at a depth of 1 mm is alwaysabove 860 HV for a temperature of solution treatment of 1100° C. and isalways above 880 HV for a temperature of solution treatment of 1150° C.for all the examples except comparative example 1 (effect of the lack ofalloying elements).

The hardnesses of the base materials are all below 650 HV.

2ND SERIES OF EXAMPLES

2 heats of 100 kg each (one example according to the invention and acomparative example 2) were produced by the VIM process according to thecomposition shown in Table 4 below (in wt % relative to the total weightof the composition), the remainder being Fe:

TABLE 4 Element C Ni Cr Mo V W Co Si Mn Al N Example 7: 0.06 3.2 3.9 4.82.1 1.1 10.2 0.16 0.14 GRADE H Comparative 0.05 3.1 3.8 5.0 2.1 2.8 10.00.17 0.14 example 2: GRADE I

These laboratory heats were transformed into bars with a diameter of 40mm by hot forging with a 2000 T press. Rods with a diameter of 20 mmwere machined from the bar and carburized.

The carburized rods were treated by the same method as for the firsttest series apart from the solution treatment, which was carried out at1100° C. and the triple tempering, which was carried out at 525° C. for1 hour. Table 5 below gives the results of the toughness tests performedon test specimens CT10 according to standard ASTM E399-17 published inFebruary 2018.

TABLE 5 Toughness Mechanical Example (MPa · √m) strength (MPa) Example 744-60 1400-1700 Comparative example 2 35 1500

Comparative example 2 has delta ferrite after heat treatment, at a lowlevel but sufficient to decrease the toughness properties.

Example 7, very close to comparative example 2 at the level of itscomposition apart from W, does not have delta ferrite and makes itpossible to obtain toughness values almost doubled relative tocomparative example 2 while maintaining good mechanical strength (Rm) ofabout 1500 MPa, which was determined according to standard ASTM E399-17published in February 2018, equivalent to a core hardness of 450 HVaccording to standard ASTM E384 published in August 2017.

1. A steel composition comprising, in percentages by weight of the totalcomposition: Carbon: 0.06-0.20; Chromium: 2.5-5.0; Molybdenum: 4.0-6.0;Tungsten: 0.01-3.0; Vanadium: 1.0-3.0; Nickel: 2.0-4.0; Cobalt:9.0-12.5; Iron: remainder as well as the inevitable impurities,optionally further comprising one or more of the following elements:Niobium: ≤2.0; Nitrogen: ≤0.50; Silicon: ≤0.70; Manganese: ≤0.70;Aluminum: ≤0.15; the combined niobium+vanadium content being in therange 1.0-3.5; and the carbon 30 nitrogen content being in the range0.06-0.50.
 2. The steel composition as claimed in claim 1, comprising,in percentages by weight of the total composition: Carbon: 0.06-0.20;Chromium: 3.0-4.5; Molybdenum: 4.0-6.0; Tungsten 0.01-3.0; Vanadium:1.5-2.5; Nickel: 2.0-4.0; Cobalt: 9.5-12.5; Iron: remainder as well asthe inevitable impurities, optionally further comprising one or more ofthe following elements: Niobium: ≤2.0; Nitrogen: ≤0.20; Silicon: ≤0.70;Manganese: ≤0.70; Aluminum: ≤0.10; the combined niobium+vanadium contentbeing in the range 1.0-3.5; and the carbon+nitrogen content being in therange 0.06-0.50.
 3. The steel composition as claimed in claim 1,comprising at most 1 wt % of inevitable impurities.
 4. The steelcomposition as claimed in claim 1, wherein the inevitable impurities areselected from titanium, sulfur, phosphorus, copper, tin, lead, oxygenand mixtures thereof.
 5. The steel composition as claimed in claim 1,which is carburizable and/or nitridable.
 6. The steel composition asclaimed in claim 1, which has, after a thermochemical treatment,followed by a heat treatment, a surface hardness above 67 HRC.
 7. Thesteel composition as claimed in claim 1, which has, after athermochemical treatment, followed by a heat treatment, a martensiticstructure having a residual austenite content below 0.5 wt % and freefrom ferrite and pearlite.
 8. The steel composition as claimed in claim6, wherein the thermal treatment comprises a solution treatment at atemperature between 1090° C.-1160° C. followed by quenching optionallywith cooling and several tempering operations at a temperature between475° C. and 530° C.
 9. A method of making a steel blank having thecomposition as claimed in claim 1, comprising: a) a steelmaking step; b)a step of transformation of the steel; c) a thermochemical treatment; d)and a heat treatment.
 10. The method of making as claimed in claim 9,wherein step c) consists of a treatment of carburizing or of nitridingor of carbonitriding or of carburizing and then nitriding.
 11. Themethod of making as claimed in claim 9, wherein step c) consists of acarburizing treatment allowing carbon enrichment of the surface leadingto a final surface carbon content of at least 1 wt %.
 12. The method ofmaking as claimed in claim 9, wherein step d) comprises a solutiontreatment at a temperature between 1090° C.-1160° C. followed by holdingat this temperature until completion of austenitization optionally withcooling to a temperature below −40° C., and several tempering operationsat a temperature between 475° C. and 530° C.
 13. The method of making asclaimed in claim 9, wherein step b) consists of a step of rolling,forging and/or extrusion.
 14. The method of making as claimed in claim9, wherein the steelmaking step a) is carried out by a conventionalsteelmaking process in an arc furnace and with refining and remeltingunder conductive slag (ESR, electroslag remelting), or by a VIM orVIM-VAR process, optionally with a step of remelting under conductiveslag (ESR, electroslag remelting) and/or under vacuum (VAR), or bypowder metallurgy such as gas atomization and compaction by hotisostatic pressing (HIP).
 15. A steel blank obtainable by a method asclaimed in claim
 9. 16. (canceled)
 17. A mechanical device made of steelhaving the composition as claimed in claim
 1. 18. An injection systemmade of steel having the composition as claimed in claim
 1. 19. Amechanical device as claimed in claim 17 which is a transmissioncomponent.
 20. A mechanical device as claimed in claim 17 which is atransmission component, a bearing, or a gear train.